1. Field of Invention
This invention relates to systems for handling of bulk liquids in vessels in emergencies when storage tanks for those liquids have been flooded. Bulk liquid is carried by all motor and steam vessels as bunker fuel and, in vessels designed for carriage of bulk liquid cargo-(barge or ship), as cargo.
The invention relates in particular to liquid handling systems in all classes of motor and steam vessels but is specifically described hereinafter as it is applied to petroleum cargo-carrying tankvessels.
2. Description of the Prior Art
Most state of the art bulk liquid cargo carrying tank-vessels are provided with cargo handling systems which include piping connected to a central, high capacity cargo pump system at one end and to "bellmouth" intakes fixed close to the bottom of each cargo tank for normal cargo removal. Through valves, one or more bellmouth intakes can be connected to the cargo pump system to produce suction in selected tanks for removal of cargo from those tanks for transfer to other tanks or ashore. The bellmouth intakes are located near the bottom of the tank and generally near the aft wall of the tank so that the maximum amount of cargo may be removed from the tank through the cargo pump system. With this arrangement, by trimming the vessel by her stern, the bellmouth intake will remain immersed and capable of withdrawing cargo for the maximum length of time. Cargo remaining is then removed by a lower capacity cargo stripping system.
Another class of tankvessels, the chemical-carrying parcel or "drugstore" ships routinely carrying a variety of different chemicals as cargo. Because these different cargoes require segregation at all times, the cargo system of these vessels is designed with individual piping systems to each tank and individual pumping systems for each tank's piping. The systems, utilize "deep well" submersible pumps located in the bottom of the tank.
When a loaded tankvessel is collided with or goes aground with a hull rupture in way of her cargo tanks, seawater floods into a ruptured tank displacing the cargo in the tank as a function of the level of the highest point of damage in the hull. Where the cargo is lighter than the seawater, as the majority of liquid cargoes are, the displaced cargo, discharged to the sea as a spill, flows out rapidly at first and then gradually decreases in flow rate as the water/cargo interface approaches a natural equilibrium based on relative head between the cargo and sea. This natural equilibrium or "water bottom" is unstable, easily upset by relative motion between the vessel and the sea. Continuing spillage will normally result from wave and current action or from further movement or motion of the ship.
If the cargo system were to be adapted to function in each tank, regardless of the flooded condition of that tank, the size of the spill could be significantly reduced since cargo could be transferred from the holed tank(s) before equilibrium is reached. It would also be of great help if the cargo in the holed tank(s) could be transferred or reduced to a degree that the resulting natural equilibrium is stabilized to prevent further spillage from the vessel caused by relative motion between the vessel and the sea. Such stabilization would also allow more radical movement of the vessel to speed her salvage and removal from further peril.
Because of the configuration of present cargo handling systems, however, the water bottom in a flooded tank will envelope the bellmouth or submersible pump suction intakes, effectively disabling the vessel's cargo pumping system in the flooded tank. Stranded tankers have therefore been required to await the arrival of emergency "over the top" pumping equipment to have cargo in their holed tanks removed or reduced. This has resulted in the loss of all of the cargo whose discharge was required to achieve a natural interface and has involved a substantial delay in rectifying a casualty, prolonging the spillage of cargo and endangering the vessel and remaining cargo by delaying salvage efforts until natural equilibrium in the holed tanks is stabilized. In the EXXON VALDEZ grounding in Alaska in 1989, several days passed before sufficient emergency "over the top" pumping capacity arrived and was rigged to stabilize the several water bottoms created in that vessel by the grounding, substantially prolonging the duration (and increasing the ultimate quantity) of spillage from the vessel and the length of time she remained on her strand and vulnerable to the elements.
A device intended to accomplish similar objectives to those of the present invention is disclosed in U.S. Pat. No. 4,389,959. This device proposes a system to offload cargo from a holed vessel tank by means of an additional, independent cargo handling system fixed at a designated distance above the hull bottom and the vessel's indigenous cargo handling system. Aside from the added expense, weight and increased complexity of a system requiring an additional cargo handling system with redundant piping, cargo pump and valving, this system also introduces increased maintenance effort and increased probabilities of failure inherent in a system that would normally be used only in an emergency.
As was discussed above, the inventor is also aware of chemical-carrying "drugstore" ship tankvessels in standard operation today which utilize individual submersible pumps in cargo tanks for cargo handling. These systems, with their pumps fixed and relatively immobile, cannot function in the manner of the present invention.